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Microwave heating is a proven technology in many industries as it increases speed and energy savings. However, when it comes to the dental industry, accuracy has proven to be an issue as demonstrated below. Tests, similar to the following, can be easily reproduced and verified by any lab. Generally, single crowns work fine for microwave sintering, but when it comes to any type of bridge cases, complete positive seating will be adversely effected.
B & D Dental spent considerable effort on the development of a microwave sintering oven for dental applications and worked closely with beta test laboratories, before ultimately deciding not to release the oven due to limitations of the microwave technology for dental applications. The subsequent focus on the more accurate and reliable coil technology gave birth to the DuoTon sintering oven. |
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| We CAD designed 10 small zirconia plates using Solidworks software and produced STL files to mill identical parts with evenly finished surfaces. They were 40mm long, 10mm wide, and 2mm thick. Two hour sintering was applied to 5 pieces each using the Coil Method and Microwave Method to see the effect of fast sintering. The DuoTron was used for the coil sintering tests and one of the commercially available microwave ovens developed for sintering of dental zirconia was used for the others. |
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| Result: Sintering was successfully finished for both methods. The plates sintered in the DuoTron oven maintained a uniform and consistent shape, while the plates in the microwave oven came out bent with irregular and inconsistent forms. |
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| A four-unit posterior bridge was designed with the 3shape CAD software and two Identical bridges were milled from the file out of one 98mm disc. The material was iso-statically pressed zirconia. This material is regarded to be one of the highest quality and consistent materials available in the market. |
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| Result: After duplicating the test 5 times, the following results were observed. the Bridge sintered with the coil method showed perfect seating for each individual die as well as the bridge framework as a whole. The bridge sintered in the microwave, however, had extremely open margins. Die #28 seated ok individually, with #29 not seating well at all, and #31 was quite distorted. Overall, the relationship of the framework #29 - 31 does not correspond well with the stone model/dies. This is in line with the uneven shrinkage of the zirconia found in test 1 above. It is noticeable that the pontic area (a relatively large mass of material) affects the seating negatively due to the distortion from the microwave sintering method. |
When sintering zirconia, especially in the case of bridges (with pontics - large masses) the heat source must be multi-directional and must be able to provide consistent and balanced heat.
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